This invention relates to antenna windshields and particularly to those of laminated glass or other rigid transparent glazing material such as polycarbonates, acrylics and polyesters, laminated to a layer of flexible plastic that has an elongated electroconductive element embedded therein. The electroconductive element is preferably a wire covered by a moisture-impervious, non-electroconductive sheath. The wire is adapted to be coupled to a radio in a vehicle in which the windshield is installed so that the wire serves as an antenna for the radio.
Laminated antenna windshields comprising a pair of matched glass sheets laminated to opposite sides of an interlayer of polyvinyl butyral in which one or more electroconductive wires are embedded in the interlayer have served as antennas for radios installed in automobiles. Using the laminated windshield to support the antenna has improved the styling of automobiles by eliminating the whip antenna that extended from the vehicle body. Its use has also avoided a source of vandalism--the snapping of the antenna from the automobile.
Several patents have been issued on laminated antenna windshields. These include U.S. Pat. No. 3,208,070 to Boicey, U.S. Pat. Nos. 3,484,583 and 3,484,584 to Shaw, U.S. Pat. Nos. 3,543,272 and 3,638,225 to Zawodniak, 3,549,785 to Timko, 3,576,576 to Jensen, 3,579,243 to Dickason, 3,599,214 to Altmayer, 3,615,986 and 3,618,102 to Dickason and Richardson, 3,680,132 to Tolliver, and 3,728,732 to Igarashi. All of the aforesaid patents illustrate laminated antenna windshields having antenna wires embedded in an interlayer of polyvinyl butyral which is sandwiched between a pair of glass sheets.
The Tolliver patent, assigned to the same assignee as the present case, suggests using polyurethane or polyvinyl butyral as an interlayer material in an antenna windshield, but does not specify any particular position of the antenna wire relative to the inner and outer glass sheets. Furthermore, none of the patents in the laminated antenna windshield art either suggested or made obvious a laminated antenna windshield of the so-called bilayer type where one layer is composed of a sheet of glass or other rigid transparent glazing material and the other layer is composed of a preformed sheet of thermoplastic polyurethane into which an elongated antenna element is embedded in close adjacency to the glass sheet.
A suitable measurement of the performance of an antenna windshield is its so-called Q-value. The Q-value is an important characteristic of any tuned circuit and is defined as the ratio between the amount of energy stored in the circuit and the amount of energy lost. A standard test has been devised using a Q-meter tuned to a frequency of 1 megahertz. The Q-meter is connected in place of the automobile radio to a so-called pig tail wire electrically connected to the antenna wire of the antenna windshield. A commercially available Q-meter suitable for use in this standard test is manufactured by Hewlett Packard and sold under the Boonton Radio trademark as type 260-A.
Each antenna windshield tested is mounted in the frame of an automobile so that the windshield occupies the same position it would normally occupy when permanently installed in the automobile. The Q-meter test has been used to evaluate many changes proposed in the past for antenna windshield configurations. As will be disclosed later in this specification, the teaching of the present invention has resulted in antenna windshields having higher Q-values and better radio reception than conventional laminated antenna windshields composed of two glass sheets and an interlayer of polyvinyl butyral.